Christopher starts counting at 39, and he counts by fours. If 39 is the 1st number that Christopher counts. what is the 7th number that he counts?
Solution: What is the first number that he counts? $39$ What is the second number that he counts? $\begin{align*}&39 + 4 \\ &= 43\end{align*}$ What is the third number that he counts? $\begin{align*}&39 + 4 + 4 \\ &= 39 + (2 \times 4) \\ &= 47\end{align*}$ What is the 7th number that he counts? $\begin{align*}&39 + (6\times4) \\ &= 39 + 24 \\ &= 63\end{align*}$